In a three dimensional image process, a picture obtained by looking at a virtual three dimensional space where an object is placed from a given viewpoint is displayed for output. For the above, a technique referred to as bump mapping is available for expressing concave and convex features on a surface of an object. According to bump mapping, the normal vector of an object surface is modified according to the concave and convex feature so that shadow (light and shade) in accordance with the concave and convex feature can be imparted to the object surface. That is, use of bump mapping can impart shadow (light and shade) in accordance with concave and convex features to an object surface without strict expression of the concave and convex features on the object surface, using polygons.
According to typical bump mapping, a normal vector (hereinafter referred to as a “pseudo-normal vector”) is hypothetically set on each of the pixels for a texture image to be mapped onto an object surface, and a shading process (a process for imparting shadow in accordance with concave and convex feature to an object surface) is carried out to the object surface, based on the pseudo-normal vectors set on the respective pixels. In the above, data referred to as a “normal map” is generally prepared as data indicating a pseudo-normal vector for each pixel of a texture image to be mapped onto the object surface. A normal map is data expressing a pseudo-normal vector set for each pixel of a texture image in the form of an RGB value. That is, a normal map is data expressing a WX axial direction component in the virtual three dimensional space (or a space comprising the WX, WY, and WZ axes) in the form of an R (red) value, a WY axial direction component in the form of a G (green) value, and a WZ axial direction component in the form of a B (blue) value. FIG. 12 is a diagram outlining the shading process. In the shading process, in rendering a polygon 54 which forms an object, the brightness of each pixel 52 is determined based on the pseudo-normal vector N of the pixel 52. More specifically, the brightness of each image 52 is determined such that when the angle θ formed by the light source direction vector L indicating the direction from the pixel 52 to the light source 50 and the pseudo-normal vector N of the pixel 52 is smaller, higher brightness results for the pixel 52, and when the angle θ is larger, lower brightness results for the pixel 52. This arrangement can impart fine shading on an object surface.
Patent Document 1: JP2001-283250A
Patent Document 2: JP2004-102900A